In: Biology
Ovary produces female sex hormones such as oestrogen and progesterone. The secretion of these hormones are controlled by follicle - stimulating hormone (FSH) and lutenizing hormone (LH) which are produced by pituitary gland.
Oestrogens are steroidal hormones secreted by the theca interna of the ovum, cells of graffian follicle, corpus luteum and placenta. Oestrogen hormone controls the development of accessory sex organ and secondary sexual characters. It regulates menstrual cycle. Oestrogen promotes growth of ovarian follicles. Further it causes the growth of the breasts and formation of melanin pigments.
Progesterone is the principal hormone secreted by the corpus
luteum. It prepares the uterus for implantation of the fertilized
ovum.It helps in the formation of placenta and for maintaining
pregnancy. Further during preguancy it
inhibits contraction of uterus. It also suppresses ovulation and
menstruation in pregnancy.
The follicular phase is initiated by the release of follicle stimulating hormone (FSH) from the anterior pituitary gland. During this phase, a primary follicle begins to enlarge. While it is growing, the follicles secrete oestrogen and smaller quantities of progesterone into the blood. Under the effects of oestrogen from the developing follicle, the endometrium starts regenerating from the basal portion. Deeper parts of the glands start proliferating. By day 14, graffian follicle has matured and pushed up against the ovary surface. The secretion of FSH ceases at the end of follicular phase.
The luteal phase begins at day 14. Under the influence of
Leutenizing hormone (LH) released by the anterior pituitary,
rupture of graffian follicle and release of ovum (ovulation)
occurs. After ovulation, the empty follicle is
transformed into a transitory endocrine gland called corpus luteum.
The corpus luteum slowly increases in size and releases a large
quantity of progestrone and smaller amount of oestrogen into the
blood. The progesterone acts on endometrium of the uterus,
preparing it to receive the fertilized ovum. To maintain pregnancy
and to prevent the contraction of uterus, the progesterone hormone
is highly essential. If there is no fertilisation, the corpus
luteum degenerates and is reabsorbed by the ovary at the end of
luteal phase.
The decline in progesterone and oestrogen initiates shedding of the endometrium and severe bleeding in a process called mensus or menstruation. During this phase, all the extra layers are sloughed off along with unfertilized egg. At the termination of menstruation, the corpus luteum is converted into a scar tissue called corpus albicans.